Cement accelerator and method

ABSTRACT

An alkali-free accelerator for sprayed cementitious composition, such as concrete, is prepared by dissolving aluminium sulphate in water and amorphous aluminium hydroxide in water optionally containing amine, and adding at least one stabiliser which is glycolic acid, and optionally at least one defoaming agent.

This invention relates to cementitious compositions and to acceleratorsfor use therein, particularly for sprayed concrete.

The application of a cementitious material such as concrete to asubstrate by spraying from a nozzle is a well-established technology,and it is widely used in such applications as the lining of tunnels. Itis important that the sprayed concrete set very rapidly on thesubstrate, and this is achieved by the addition to the concrete at thenozzle of an accelerator. These accelerators are quite different fromthose used with conventional concrete and have traditionally includedsuch materials as alkali metal hydroxides, aluminates and silicates.

The highly alkaline nature of these materials has given handingproblems. It also means that their use in confined spaces such astunnels has led to very unpleasant working atmospheres. Recent attemptsto avoid such materials have involved the use of aluminium compounds andtypical examples may be found in European Patents 0 076 927, 0 775 097and 0 742 179, Australian Patent 706917 and European Applications 0 812812 and 0 946 451.

It has now been found that it is possible to prepare an accelerator forsprayed concrete by a simple process, which accelerator performsespecially well. The invention therefore provides a method of preparingan accelerator for sprayed cementitious material consisting essentiallyof the steps of

-   -   (i) dissolving aluminium sulphate and aluminium hydroxide in        water, which optionally contains at least one amine dissolved        therein, to give a clear solution; and

(ii) optionally adding at least one defoaming agent;

the proportions of ingredients present being such that the final productcontains from 3%-12% by weight of aluminium sulphate (measured asAl₂O₃), up to 30% by weight of amorphous aluminium hydroxide, up to 15%by weight amine, up to 3% by weight defoaming agent and up to 1.5mol/kg. stabiliser, the stabiliser being glycolic acid.

The invention additionally provides an accelerator for use with sprayedcementitious material preparable by such a method.

The invention additionally provides an accelerator for cementitiousmaterial consisting essentially of a solution of aluminium sulphate,amorphous aluminium hydroxide and glycolic acid, optionally containingamine and defoaming agent, the accelerator containing from 3%-12% byweight of aluminium sulphate (measured as Al₂O₃), up to 30% by weight ofamorphous aluminium hydroxide, up to 15% by weight amine, up to 3% byweight defoaming agent and up to 1.5 mol/kg. glycolic acid.

The aluminium sulphate used may be any commercially-available material.Aluminium sulphates differ in their purity and constitution, the mostcommon being so-called “17%” because it contains 17% of Al₂O₃. Inpractical terms, the weight percentage of 17% aluminium sulphate,Al₂(SO₄)₃. 14.3H₂O, which should be used in the process according to theinvention lies in the range of from 30% to 60%, preferably from 40%-54%.

The aluminium hydroxide may be any commercially-available amorphousaluminium hydroxide. Although all such aluminium hydroxides will givesatisfactory results, it is generally true that the more recent the dateof manufacture, the better the result. In addition, aluminium hydroxideswhich, as a result of their particular manner of manufacture, contain asmall proportion of aluminium carbonate (up to 5%) are easier todissolve and are preferred materials. This behaviour is not obtained bysimply adding aluminium carbonate to pure aluminium hydroxide. Althoughvery small quantities of aluminium hydroxide may be used (less than 0.1%is possible), a significant improvement is observed at 5% or more. Thepreferred range of weight proportions is from 4-25%, preferably from6-18%.

There must also be present a stabiliser, which may be added at the endof the process. This is a material which prevents the aluminiumhydroxide/aluminium sulphate solution either from precipitating or fromforming a gel. Without stabiliser, the solution will function well as anaccelerator, but it will lack stability and therefore shelf life,necessitating its use very shortly after manufacture, something usuallynot practical.

The stabiliser for use in this invention is glycolic acid (hydroxyaceticacid). It has been previously proposed to use hydroxycarboxylic acids asstabilisers in International Application PCT/EP00/12216, but there wasno specific mention of glycolic acid and the preferred acid in thatapplication was citric acid. It has been found surprisingly thatglycolic acid gives outstanding stability, far better than otherhydroxycarboxylic acids. Preferably the stabiliser is present to theextent of from 0.1-1 mol./kg. As a general rule, the more glycolic acidpresent, the better the stability. However, there comes a point whenaddition of further glycolic acid brings no improvement. Thus, althoughthe stated maximum of 1.5 mol./kg. can be exceeded, this brings nobenefit. The best results are obtained by using quantities in the regionof 1 mol./kg.

The properties of the accelerators provided by this invention can beconsiderably enhanced by the use of one or both of two optional, butpreferred, components.

The first of these is amine. This must be water-soluble, otherwise thereis no restriction on the choice of amine. Preferred amines arealkanolamines, such as diglycolamine, diethanolamine andtriethanolamine, diethanolamine being particularly preferred. Up to 12%by weight amine may be used, but the preferred quantity is from 1-4%.

The second preferred additional component, defoaming agent, may be anysuch material known to the art. Most of these are proprietary commercialmaterials whose precise composition is never revealed, but any suchmaterial known to the art is suitable. Typical examples include siliconetypes such as AGITAN (trade mark) and fatty acid polyether types such asLUMITEN (trade mark) EL.

The defoaming agent may be used at a rate out up to 5% (solids by weightof the whole composition), preferably from 0.5%-3%. The use of defoamingagent makes the use of less fresh aluminium hydroxides easier. It isbelieved, without restricting the scope of the invention in any way,that its presence helps in the removal of carbon dioxide whichaccumulates on the surface of the aluminium hydroxide over time.Surprisingly, provided that the defoamer contains no silicone and thatit is not present to the extent of more than 3%, it gives an appreciableimprovement in setting time over that of an identical compositionwithout defoaming agent or with silicone types.

The process of the invention is readily carried out with standardequipment, and the skilled person will have no difficulty in doing so.It will be appreciated that in order to achieve solutions at the variousstages, some heating may be necessary, typically to about 50-60° C.

In the process, the clear solution can be produced by any convenientmethod. It is possible to add the aluminium sulphate and aluminiumhydroxide sequentially in any order to water. It is also possible to addthem together to water, or to dissolve or disperse them individually intwo different quantities of water and then combine these quantities.

Preferably, the aluminium sulphate and the aluminium hydroxide are addedsequentially to water. Preferably the aluminium sulphate is firstdissolved in water; aluminium sulphate will dissolve with heating. Tothis solution the aluminium hydroxide is then added. A clear solution isobtained.

It is possible, although less preferable, first to add the aluminiumhydroxide to the water. Aluminium hydroxide does not dissolve readily inwater, but gives a fine suspension. To this suspension the aluminiumsulphate is added. A clear solution is obtained. The stabiliser is addedto this solution and stirred in.

The precise nature of the product of the process is not known. It iscertainly not a mere mixture of the original components (the fact thatthe product is a clear or slightly turbid solution and not an opaquesuspension typical of aluminium hydroxide is evidence of this), andwithout restricting the invention in any way, it is believed to beoligomeric or polymeric in nature.

The accelerator thus prepared gives excellent results when used as anaccelerator for sprayed cementitious material, especially concrete.Sprayed concrete treated with this accelerator hardens rapidly and hasgood final strength. The accelerator has a long shelf-life, is resistantto changes in temperature and is completely non-alkaline, thus leadingto better working environments.

The invention therefore also provides a method of applying acementitious composition to a substrate by spraying, comprising theconveying of the composition to a spray nozzle, there being added at thenozzle an accelerating composition of the kind hereinabove described.

The invention is further illustrated by the following non-limitingexamples.

An accelerator is made from the following ingredients: aluminiumsulphate (17%) 42 parts (weight) aluminium hydroxide 18 parts (weight)glycolic acid  4 parts (weight) diethanolamine  1 parts (weight) water35 parts (weight)

The water is heated to 55°-60° C., and the aluminium sulphate is addedwith stirring. When it has dissolved, the aluminium hydroxide is added,followed by the glycolic acids and the amine. The mixture is thenallowed to cool. The final solution is a pale yellow, slightly turbidsolution.

This accelerator composition is called Accelerator A.

Another accelerator (Accelerator B) is prepared in the same manner, butsubstituting formic acid, a well-known stabiliser, for glycolic acid.The quantity of water is reduced, so that more formic acid can beadded—the quantity of formic acid in Accelerator B is twice that ofglycolic acid in Accelerator A.

Both accelerators are subjected to long-term stability testing at 5° C.,20° C. and 40° C. The particular test period is 70 days. The assessmentcriteria and the results are as follows:

a) Change in turbidity Both accelerators show no change over the entireperiod at 5° and 20°. However, at 40°, Accelerator A shows a turbiditychange (indicating the onset of instability) at 31 days, whereasAccelerator B showed this change at 13 days.

(b) Gelling A gelled accelerator cannot be used. Both accelerators showno gelling over the whole period at 5° and 20°, but Accelerator A gelledat 59 days and Accelerator B at 27 days.

(c) Separation The separation of the accelerator solution into discreteliquid layers indicates that it can no longer be used. Both acceleratorsshow no separation over the period.

(d) Sedimentation Excessive sedimentation (in excess of 1 mm) means thatan accelerator can no longer be used. Neither accelerator has excessivesedimentation over the period of the test.

The performance of Accelerator B is typical of a high-performancecommercial accelerator. It can readily be seen that Accelerator Aaccording to the invention has a significant edge in stabilityperformance, even though it has only half of the quantity of stabiliser.40° C. is typical of the temperatures commonly encountered in tunnels,and it is often required that accelerators ready for use be stored insuch conditions. The improvement in performance shown by Accelerator Bis therefore of major commercial significance, as it saves the need forless stable accelerators to be stored in cooler conditions and thenbrought in on demand.

1. A method of preparing an accelerator for sprayed cementitiouscompositions, consisting essentially of the steps of (i) dissolvingaluminium sulphate and aluminium hydroxide in water, which optionallycontains at least one amine dissolved therein, to give a clear solution;and (ii) optionally adding at least one defoaming agent; the proportionsof ingredients present being such that the final product contains fromabout 3% to about 12% by weight of aluminium sulphate (measured asAl₂O₃), up to about 30% by weight of amorphous aluminium hydroxide, upto about 15% by weight amine, up to about 3% solids by weight of thewhole composition defoaming agent and up to about 1.5 mol/kg.stabiliser, the accelerator additionally containing a stabiliser whichis glycolic acid.
 2. A method according to claim 1, wherein there isadditionally present in the water at least one water-soluble amine.
 3. Amethod according to claim 3, wherein there is added at least onedefoaming agent.
 4. A method according to claim 3, wherein the defoamingagent contains no silicone and is present to a maximum extent of about3% solids by weight of the whole composition.
 5. A method according toclaim 1, wherein the clear solution is prepared by the steps of (i)dissolving aluminium sulphate in water, optionally containing at leastone amine dissolved therein; (ii) dissolving amorphous aluminiumhydroxide in the solution of (i) until a clear solution is obtained; and(iii) adding glycolic acid to the clear solution.
 6. A method accordingto claim 5, wherein there is added to the clear solution at least onedefoaming agent.
 7. An accelerator for use with sprayed cementitiouscompositions, consisting essentially of a solution of aluminiumsulphate, amorphous aluminium hydroxide and glycolic acid, optionallycontaining amine and defoaming agent, the accelerator containing fromabout 3% to about 12% by weight of aluminium sulphate (measured asAl₂O₃), up to about 30% by weight of amorphous aluminium hydroxide, upto about 15% by weight amine, up to about 3% solids by weight of thewhole composition defoaming agent and up to about 1.5 mol/kg. glycolicacid.
 8. An accelerator for use with sprayed cementitious compositions,produced by a process that consists essentially of the steps of (i)dissolving aluminium sulphate and aluminium hydroxide in water, whichoptionally contains at least one amine dissolved therein, to give aclear solution; and (ii) optionally adding at least one defoaming agent;the proportions of ingredients present being such that the final productcontains from about 3% to about 12% by weight of aluminium sulphate(measured as Al₂O₃), up to about 30% by weight of amorphous aluminiumhydroxide, up to about 15% by weight amine, up to about 3% solids byweight of the whole composition defoaming agent and up to about 1.5mol/kg. stabiliser, the accelerator additionally containing a stabiliserwhich is glycolic acid.
 9. A method of applying a cementitiouscomposition to a substrate by spraying, comprising the conveying of thecomposition to a spray nozzle, there being added at the nozzle theaccelerator prepared by the method of claim
 6. 10. A method of applyinga cementitious composition to a substrate by spraying, comprising theconveying of the composition to a spray nozzle, there being added at thenozzle the accelerator of claim
 7. 11. A method of applying acementitious composition to a substrate by spraying, comprising theconveying of the composition to a spray nozzle, there being added at thenozzle the accelerator of claim 8.